Beside its favorable electronic properties, silicon has become increasingly attractive as a material for manufacturing micromechanical devices (MEMS-devices). These devices offer a wide range of applications ranging from actuators to a plurality of different sensors, e.g. accelerometers for detecting a car acceleration and for triggering safety equipment such as an airbag. Often, the sensor uses electrostatic actuation and capacitive read-out for detecting a displacement of a movable component with respect to a static component of the micromechanical device.
A variety of different technologies for manufacturing MEMS-devices have been developed in the recent time. An exemplary technical approach is surface micromechanics. According to this technique, thin films are grown or deposited on a substrate or wafer surface. The silicon substrate itself merely acts as a base for the micromechanical structures while the movable parts of the MEMS-device are manufactured out of the deposited thin film material. This however leads to a maximum thickness of a few micrometers for the movable components of the MEMS-device. Further, intrinsic stress in the deposited structures or films may deteriorate these devices.
A further approach for manufacturing MEMS-devices are bulk micromechanics. The MEMS-devices are made from the typically monocrystalline silicon wafer material, preferably by etching the respective structures. The main benefit of bulk micromechanics is the large mass which is available for the movable parts of the MEMS-device. This may be advantageous, especially for the design of accelerometers. Further, the inferior electronic properties of single crystalline silicon may be of interest. However, for manufacturing active and/or passive components, a combination of different thermal processing steps is needed for determining their specific electrical properties. If a MEMS-device should be integrated in such a device comprising active and passive components, these thermal processing steps have to be changed. Consequently, both, the optimum performance for demanding new technologies comprising passive and active devices and the maximum sensitivity of micromechanical components are difficult if not impossible to achieve.
Recently, another technology for manufacturing MEMS-devices is becoming more and more attractive. Silicon on insulator (SOI) substrates or wafers which are known from the production of high-grade integrated circuits (IC) are applied for manufacturing MEMS-devices. Preferably, the monocrystalline silicon layer which is arranged on a buried silicon oxide layer is applied for manufacturing e.g. a movable component of the MEMS-device.
A further step towards the design of complex micro-electromechanical systems is the integration of an integrated circuit, i.e. an electronic device (IC-device) and a MEMS-device on a single substrate. For this appealing concept, there are different approaches for manufacturing such a system. In principle, the MEMS-device may be fabricated before the IC-device or vice versa. This however demands for complex backend integration of the two parts. If the MEMS-device is fabricated first, it has to be taken into account that a prerequisite for the later IC process is the availability of a smooth high quality single crystalline silicon surface for active device processing. Typically, the wafer surface has to be planarized. Further, the MEMS structure has to withstand the harsh semiconductor fabrication process conditions including high temperature steps, ion implantations, etching steps, etc. On the other hand, if the IC device is fabricated first, especially the metallization has to withstand high temperature annealing steps which are necessary for stress control and curing of the MEMS part. Typically, this is not compatible with the IC metallization scheme or shallow junctions which are needed in modern circuits. A further drawback of this approach is the integration of process steps and the redesign of the device to allow a release of the movable part. Large spacings or cost intensive mask steps may be necessary to perform this mechanic release of the micromechanical part without affecting the integrated circuit on the other hand.
A more sophisticated approach is a mixed process where the fabrication steps for the IC- and MEMS-device are interleaved.